The rookie, top recruit in his class, resigned after the incident, which he said was covered up. The deputy's supervisor was allegedly threatened by the young man's uncle, a sheriff's detective.

A Los Angeles County sheriff's rookie who graduated at the top of his recruit class resigned after only a few weeks on the job, alleging that a supervisor made him beat up a mentally ill jail inmate, according to interviews and law enforcement records.

The deputy, Joshua Sather, said that shortly before the inmate's beating his supervisor said, "We're gonna go in and teach this guy a lesson," according to the records. The attack, Sather said, was then covered up.

Law enforcement records reveal that the incident caused tensions in the Sheriff's Department. Sather's uncle, a veteran sheriff's detective, angrily confronted the supervisor about making his nephew "beat up 'dings,' " slang for the mentally disabled. He then allegedly threatened to "put a bullet" in the supervisor's head.

Sather's case was pieced together by The Times from department sources as well as district attorney's documents in which Sather's uncle revealed his nephew's allegations to investigators.

Sheriff's officials launched an investigation and determined that an uncooperative inmate had been subdued by force, but concluded that no misconduct had occurred. They also asked the district attorney to review the uncle's alleged threat, but prosecutors declined to file charges.

Sather's allegation is among several first-hand accounts of unwarranted deputy violence against inmates in the nation's largest jail system. Last week, two chaplains and a movie producer released sworn statements that they witnessed deputies abusing inmates. But Sather's allegations are unusual because they come from within the department's own ranks, from the point of view of a deputy.

The FBI is now investigating several allegations of deputy abuse and misconduct in the jails.

Sather, a Long Beach native, had followed in his uncle's footsteps, earning a spot in the Sheriff's Academy and becoming his class' sole "Honor Recruit" for his leadership, athleticism and other abilities. As with virtually all rookies, his first assignment was jail duty.

The jails are a place where inexperienced deputies learn how to handle potentially violent and manipulative criminals, while under constant supervision. For Sather, the experience quickly became disturbing.

Sather's beating allegation and the drama that followed his decision to resign are documented in a seven-page district attorney's memo reviewed by The Times. The following account is based on that report:

On March 22, 2010, Sather was working on the sixth floor mental health ward of the Twin Towers Correctional Facility in downtown L.A. At some point during his shift, he, his supervisor and other deputies used force on the mentally ill inmate.

Soon afterward, Sather, a muscled, tattooed 23-year-old, called his uncle, Steven Sather, crying and distraught, records show. The young man apparently told his uncle that the beating was unwarranted and then had been covered up. The elder Sather told his nephew to do the "right thing" and be honest about what occurred.

The next day, worried that the rookie might be planning to quit, Steven Sather drove to Twin Towers intent on saving his nephew's career. The uncle and his partner parked behind the jail and left their guns in the trunk. In the watch commander's office, Steven Sather asked to see his nephew. But he was told the young man had resigned, citing "family issues" as his only reason.

Outside, Steven Sather and his partner had a colleague radio for Joshua Sather's supervisor, Bryan Brunsting.

When Brunsting saw the two gang detectives waiting for him, he asked what was going on.

"Do you know who I am?" Steven Sather asked.

Brunsting peered at the name embroidered on the detective's green-and-gold sheriff's raid jacket. He realized he was talking to his trainee's uncle. The rookie had failed to show up for a morning briefing, and Brunsting had just learned he'd quit.

Steven Sather led Brunsting away for a private conversation. Deputies nearby recalled that the exchange was heated.

"If you don't stop [messing] with my nephew, I'm gonna kick your ass. Stop [messing] with my nephew," Steven Sather shouted. "You know what this is about, getting him into situations that he shouldn't have got into. He's a … honor recruit and you put him into situations that you shouldn't have put him in."

Brunsting said afterward that the detective threatened him, saying, "If my nephew doesn't decide to come back to the department, I'm gonna find you and put a bullet in your head."